1. Field of the Invention
This invention pertains to a process for hydrogenating semiconductor materials. More particularly, a process to deliver hydrogen to selected areas of semiconductor structures by radiating the selected areas with ultraviolet radiation while in hydrogen is provided.
2. Description of Related Art
Within crystalline materials hydrogen interacts with broken or weak covalent bonds, such as those found at extended and localized defect centers. The main results of these interactions are shifts of energy levels out of (or into) the gap. The shift in the energy levels can lead to the passivation of the electrical activity of various centers. The consequences of these interactions are substantial changes in the electrical and optical properties of the materials and in the carrier lifetime.
Primarily for these reasons, hydrogen has been used extensively to influence the electrical properties of Si and III-V materials, as well as a number of heterostructure systems, such as GaAs/Si, GaAs/InP, and epitaxial/homoepitaxial InP/InP, among others (e.g. hydrogenation is performed as the last step of polycrystalline-silicon solar cell processing, significantly improving the performance of these cells).
There is a significant demand for improved detectors across the infrared (IR) spectrum, particularly in terms of increased spectral range, pixel sensitivity, pixel density and functionality (e.g. multi-spectral sensors). Many types of systems have been or are being developed that include IR detectors based on Schottky barriers on silicon, extrinsic Si, lead tin telluride, SiGe heterojunctions, AlGaAs multiquantum wells, InAs/InGaSb strained layer superlattices, and high-temperature superconductors. However none of these can compete with HgCdTe in terms of fundamental properties and quantum efficiency. While some of these materials may presently have more manufacturability, they will never provide a higher performance, or, with the exception of thermal detectors, operate at higher or comparable temperatures. In addition, very few systems can compete with HgCdTe simply in terms of the spectral range of operation, since its bandgap can be continuously adjusted across the corresponding IR spectral range by varying the alloy composition (Hg to Cd ratio). Thus, HgCdTe can be used for sensors with cutoff wavelengths ranging from short wavelength or near infrared (NIR, SWIR: 1-2 μm), medium wavelength (MWIR: 3-5 μm) to long wavelength (LWIR: 8-12 μm), and very long wavelength (VLWIR: 12-16 μm). For these reasons, HgCdTe is claimed to be the third most technologically important semiconductor after Si and Ge.
All device grade HgCdTe is now thin-film and grown by liquid-phase epitaxy (LPE) or molecular-beam epitaxy (MBE). However, the quality of epitaxially grown HgCdTe may suffer due to poor substrate quality. Defects within the active regions of HgCdTe devices lead to tunneling dark currents even during low temperature operation, i.e. an operability limitation of the focal plane array (FPA).
HgCdTe diode arrays also suffer from problems related to the lack of a suitable lattice-matched, large-area growth substrate. Due to the availability of large area and low cost substrates, Si is considered to be, by far, the most promising substrate for the next generation of HgCdTe devices. However, its 19% lattice mismatch with HgCdTe presents a significant technological hurdle since it leads to additional defects during growth that degrade the performance of HgCdTe devices. Hydrogen has been demonstrated to passivate these defects. Thus, hydrogenation appears to solve many of the current problems related to HgCdTe devices. However, in order to realize this benefit, cost effective hydrogenation processes must be developed.
To date, hydrogenation has always been achieved using a glow discharge technique (or a very crude approach such as boiling in water). There are no reports known to the inventors of hydrogen incorporation into semiconductors or any materials using UV and hydrogen gas.
Reports on studies of hydrogen passivation in Te-based II-VI's are limited. To the inventors' knowledge, there are only three reports of hydrogenation of HgCdTe (all of which are commented on below), and none of these reports is directly related to dislocation passivation. The studies do provide some insight into hydrogenation effects on vacancies and impurities—something of obvious concern—and also provide an initial baseline for hydrogenation parameters.
There has been work by Hughes et. al. (W. C. Hughes et al, J. Elec. Mat. 22 (8), p. 1101, 1993) on hydrogen interactions (and hence relevant to passivation) in bulk HgCdTe, by using perturbed γγ angular correlation (PAC). This work suggests that hydrogen acts mainly as an acceptor (when introduced from boiling water), interacts strongly with the In-Vacancy acceptor (in HgCdTe doped with In), but not with the bare In donor. There is also a report by Chen et. al. (Y. F. Chen et al, Appl. Phys. Lett. 59 (6) p. 703, 1991) on hydrogen passivation of bulk HgCdTe. Passivation was by plasma discharge and boiling in water. While the authors claim that Hg vacancies can be effectively passivated by atomic hydrogen, and that residual impurities or defects can be passivated in HgCdTe, the study was limited to infrared transmission spectra before and after hydrogenation. The study was inconclusive
Cheung et. al. (J. T. Cheung et al, J. Vac. Sci. Tech. B10 (4), July/August, 1992) have briefly documented the effects of various gaseous species in the presence of UV light on the dark currents in HgCdTe short-wavelength infrared diodes and observed a detrimental change in the diode characteristics when they were exposed to hydrogen and UV radiation simultaneously. The authors surmised that the anomalous behavior of the UV-induced degradation of the diode characteristics in a hydrogen atmosphere was possibly due to several mechanisms. One possibility put forward was the continuous depletion of Hg and Te from the surface due to its reaction with atomic H. This process would thus create a Cd-rich layer which serves as an electrical shunt across the junction. Another possibility involved diffusion of atomic hydrogen into the bulk to alter the junction characteristics. In this instance it was believed that it could only be produced by UV-induced heterogeneous dissociation of hydrogen on the surface, since there was no Hg vapor to act as an energy sensitizer.
The finding by Cheung provided a hint of a method of introducing hydrogen into the HgCdTe epilayers. As disclosed below, the UV light can indeed lead to incorporation of hydrogen into the HgCdTe epilayer. However, we have discovered that the diode's degradation is a result of modification to the surface states enabling a leakage current to flow along the surface and not due to the hydrogen within the junction, or with Hg and Te depletion. The work by Cheung was on HgCdTe diodes grown on lattice matched ZnCdTe substrates.
What is needed is method and apparatus for passivating defects in semiconductors such as HgCdTe, such as those defects that arise from epitaxial growth of the semiconductor layers on a substrate that is not lattice-matched to the HgCdTe, and for realizing other benefits resulting from hydrogenation, including those benefits from hydrogen in improving electrical, optical and other properties.